That looks real nice!
I had used aluminum tubes for the pommel on my aluminum /hardware saber. The pommel took more work than the rest of the saber combined. Especially the pommel cubes and the parts around it. Originally I had thought of using the neck of a metal bottle, but the shape was not good enough and the thin walls made it difficult to fit with other parts without using putty.
I have built my pommel around a 1 inch tube that goes all the way from being the silver tube inside the black grip, being the "neck" to the pommel and down to the endcap. However, if you want to make it more accurate, you should make the silver tube a little bit smaller than one inch in diameter.
For the end cap. you could use a round-headed bolt, 1 inch in diameter. It has the perfect shape. I had shaped the end of some round bar stock and cut it off, though.
Be sure to ask Roman for a red thumbscrew/button, if you don't already have one, as he is doing a run now. If you wait a little bit longer, you might miss it.